1. Field of the Invention
The present invention generally relates to digital communication systems, and more particularly to methods for signal quality measurements on Gaussian frequency shift keying (GFSK) signals.
2. Description of the Related Art
For communication systems, received signal quality measurements are important for several reasons. One of the reasons is the production line testing (PLT). PLT is required to determine whether every component in a transmitter (Tx) and a receiver (Rx) works correctly. In order for PLT to be passed and product shipped, all the measured signal qualities must meet the product specifications. Another reason is that this measured signal quality may be used for an indicator of the current physical channel condition including the Tx and the Rx distortions. This signal quality measured may be widely utilized. For example, by measuring the signal quality, the supported data rates for a current wireless channel can be easily accessed and adjusted. Or, Tx power can be controlled by this signal quality measured. This signal quality may be also utilized for packet loss concealment with sound transmitted. Not only lost packets but also bad packets with errors may be replaced for a better sound quality.
Received signal-to-noise ratio (SNR) is one of the possible indicators for decoded signal qualities. However, received SNR alone may not be a valid signal quality indicator, especially for wireless systems, as interference may also affect the signal quality. That is why signal to interference-plus-noise ratio (SINR) may be a better indicator for wireless systems. Unfortunately, it is not simple to calculate the received SNR or SINR because the received signal is a combination of a signal, an interference and a noise. The signal, the interference and the noise are hard to separate since they are unknown until the information bits are decoded correctly at the Rx. Because of this difficulty, sometimes SNR (or SINR) is approximated by the error vector magnitude (EVM). The EVM is the average distance between the received signal and the closest constellation points. It is much simpler to measure the EVM because closest constellation points can be instantly determined with simple operations. The EVM and SNR (or SINR) are not identical, but closely related. That is, the EVM is also a good indicator for decoded signal qualities.
Bluetooth [Specification of the Bluetooth System, 2.0+EDR, 4 Nov. 2004] defines differential EVM (DEVM) for enhanced data rates which adopt differential phase-shift keying (DPSK) as a modulation scheme. However, the EVM is hard to derive from modulations like Gaussian frequency shift keying (GFSK) because of its non-linearity property. Moreover, the Bluetooth GFSK signals may have different modulation index h varying from 0.28 to 0.35. Here, an efficient method to measure the signal quality of a GFSK signal with varying h is proposed.